Distortion reducing system in modulated amplifiers



Oct. 18, 1938. w. H. WIRKLER ,1

DISTORTION REDUCING SYSTEM IN MODULATED AMPLIFIERS Filed June 15, 1935 3Sheets-Sheet 1 .2 El RFINPuT AAAAA ggrpur Bms Ami/PUT MLTER H W/R/(LEROct. 18, 1938.

DISTORTION REDUCING SYSTEM IN MODULATED AMPLIFIERS w. H. WIRKLER2,133,410

Filed June 15, 1935 3 Sheets-Sheet 2 nfout ut d I 1 yr/d ML TEA H.'W/RKLER GRMWZM Patented Oct. 18, 1938 UNITED STATES PATENT OFFICEDISTORTION REDUCING SYSTEM IN MODU- LATED AMPLIFIERS Application June15,

6 Claims.

My invention relates broadly to transmitting systems and moreparticularly to circuits for substantially decreasing distortion in theoutput of modulation systems and audio frequency amplifiers in speechfrequency systems.

It is often desirable to use either grid-modulated amplifiers or class Blinear amplifiers instead of the more usual plate-modulated amplifier inradio-telephone transmitters. One objection to the use of either gridmodulation or the class B linear amplifier is that the amplitudedistortion is greater than in the plate-modulated amplifier, especiallyif the design or adjustment of the circuit used is not quite correct.Because the grid-modulated amplifier is much simpler than the linearamplifier; especially when the necessary modulating equipment for thelow level stage used to excite the linear amplifier is considered, andbecause the number of adjustments for the grid-modulated amplifier arefewer, it is desirable to reduce the distortion of the gridmodulatedamplifier so as to make its use in high quality broadcast transmitterspractical.

One of the objects of my invention is to provide a speech amplifiersystem having means for reducing the distortion in grid-modulatedamplifiers.

The main source of amplitude distortion in a grid-modulated transmitterof proper design is the non-linear relation between the input gridvoltage and the radio-frequency output current. This is a result partlyof the mechanics of grid modulation. In addition, the non-lnear tubecharacteristics accentuate this non-linearity. Further distortion isintroduced by the fluctuation of the radio-frequency excitation voltagecaused by the non-linearly varying grid current load drawn by themodulated stage during modulation. The distortion caused by all of thefactors above can be decreased to a minimum by means of the compensatingsystem of my invention.

Another object of my invention is to provide means for compensatingvariations in factors in a modulating circuit, which changes producedistortion, by withdrawing a portion of the distorted output current toa separate channel, separating the distortion elements from themodulated output, and reintroducing the distortion elements 50 into themodulating circuit in substantial opposition to the variations producingthem.

A further object of my invention is to compensate for variations infactors in a modulation circuit, which produce distortion, by divertinga portion of the modulated radio frequency output 1935, Serial No.26,830

current, rectifying this current in a linear rectifier device, opposingthe signal component in the distorted rectified current by the inputsignal current, and reintroducing the remaining distortion componentsinto the modulating circuit in substantial opposition to the variationswhich produced them.

A still further object of my invention is to provide substantiallylinear operating devices for deriving the distortion components from thedistorted output of a modulated amplifier without modification thereofso that the distortion components may be reintroduced into themodulating circuit in opposition to the causes producing them.

Still another object of my invention is to provide filter and blockingmeans disposed throughout the circuit to provide selective paths foraudio and radio frequency currents in order to isolate the distortioncomponents of a distorted modulated output so that the distortedcomponents may be employed substantially unaltered to oppose the forcesproducing them.

A still further object of my invention is to reduce the amplitude andharmonic distortion inherent in modulation systems and audio amplifierswhen operated under certain conditions.

Other and further objects reside in the circuits and arrangementshereinafter more fully described with reference to the accompanyingdrawings in which:

Figure l is a schematic diagram of the circuit connections in thedistortion reducing system of my invention as applied to agrid-modulated amplifier; Fig. 2 is a graph showing the normal deviationof the output current from the input voltage in a non-linear amplifyingdevice; Fig. 3 is a trace of an oscillogram showing the distortedoperating condition in a grid-modulated amplifier without thecompensating system of my invention; Figs. 4, 5 and 6 are traces ofoscillograms showing the operating conditions in different portions ofthe circuit shown in Fig. 1; Fig. 7 is a schematic diagram of thecircuit connections in a modified form of my invention employing anadditional amplifier stage in the compensating circuits; and Fig. 8 is asimilar diagram showing the distortion decreasing system of my inventionas incorporated in the intermediate amplifier of a radio frequencytransmitter.

Inasmuch as modulation systems and audio amplifiers are often capable ofdelivering relatively high outputs, but only when operated underconditions which would ordinarily result in high amplitude distortion,the utility of a device for reducing this distortion is at onceapparent,

as then such modulation systems and audio amplifiers can be operated atgreatly increased efficiency with consequent saving in apparatus andoperation costs.

The inherent distortion in the grid-modulated amplifier and thedistortion caused by the fluctuating radio frequency grid voltage can beeffectively reduced at the source by means of the circuit shown inFig. 1. Reference character I designates the grid-modulated stageexcited by capacity coupling to the tank circuit 2 of a preceding radiofrequency amplifier stage. Modulating voltages of audio frequency areintroduced through the transformer 8, the secondary winding of which isconnected with the grid circuit of the modulating amplifier I throughradio frequency choke coil 9. Reference character 3 indicates a lineardetector coupled to the output circuit of the modulated amplifier Ithrough the circuit 4. The rectified output of detector 3 produces avoltage across a resistor 5, connected in circuit with the detector 3,which is always proportional to the instantaneous radio frequency outputcurrent of the modulated amplifier.

A source of bias potential I 0 is connected in series with resistor 5 inopposite polarity with respect to the voltage produced across theresistor 5. Reference character l2 designates an electron tube amplifierhaving the grid l2b thereof connected through radio frequency filteringmeans H! to the cathode of the linear detector 3. The circuit l4,comprising radio frequency choke and by-pass devices, is operative tosmooth out the radio frequency component of the rectified wave emanatingfrom the detector 3 and pass the audio frequency components to the tubeI 2. The cathode of tube I2 is shown energized by alternating currentand having the usual balanced connection therefrom made to the highpotential side of the modulating voltage input transformer 8. The anodeI2a of tube I2 is connected with coupling coil l5 and radio frequencyenergy absorbing means I 6 in circuit to the oathode l2c. This circuitis paralleled by a radio frequency choke coil I! connected directly fromthe anode l2a to the cathode 120.

If the modulation system I were free of dis tortion caused bynon-linearity in the operation of the modulating amplifier, that is, ifthe ampli tude of the modulated wave varied linearly with the modulatingvoltage introduced into the grid circuit by the transformer 8, theremight still remain distortion caused by the varying impedance of thegrid circuit of the modulating amplifier. The coupling to the lineardetector 3 could be adjusted so that its audio frequency output voltageeffective in the tube I2 would equal and balance the effect of thatintroduced by transformer 8; and the bias voltage introduced by thesource I!) could be adjusted to exceed slightly the direct currentcomponent of voltage developed by the linear detector 3, thus providinga suitable negative bias for the grid of vacuum tube l2. Since thecathode I20 of tube [2 is connected to the ungrounded side of themodulation transformer 8, it will vary in potential at audio frequencyin phase with the grid Nb of tube l2. If the coupling of the lineardetector is properly adjusted, the audio frequency potential differencebetween the grid I21) and the cathode I20 of tube l2 can be made zero,so that tube l2 has only a constant negative bias effective in its gridcircuit insofar as the audio frequency voltages are concerned; and itsplate circuit, which is inductively coupled to the output circuit of theradio frequency exciting amplifier through coil 15 effects a constantload for the exciting amplifier by inductive absorption of radiofrequency energy in varying proportions as the radio frequency currentin the coil 2a in the tank circuit 2 varies.

With the adjustments made as above, on the assumption that nonon-linearity exists between grid voltage and radio frequency outputcurrent of the moduated stage, the action of the circuit when suchdistortion does exist can be best analyzed by means of an cscillograph.The oscillogram pictured in Fig. 3 has been enlarged in Fig. 2, theordinates thereof representing radio frequency output voltages and theabscissae, instantaneous grid voltages. The solid line shows the actualmodulation characteristic, without distortion compensation, obtained bymodulating an amplifier consisting of a single type 211 triode, excitedby a radio frequency amplifier consisting of a pair of type 46 triodes,and artificially loaded by means of a resistor in its output circuit.The curvature of this characteristic is plainly evident.

Applying now the compensation circuits of my invention to the systemjust outlined, the straight dotted line drawn on this same figure (Fig.2) represents the voltage output from linear detector 3 that is requiredto neutralize the audio frequency voltage, as supplied from transformer8, in the grid circuit of tube I2 at every instant during the audiofrequency cycle. It is evident that this neutralization occurs at onlythe three points marked 0 on the modulation characteristic. At a pointon the characteristic at which the difference between the two lines isdi as indicated by the arrow, the linear detector voltage is less thanthat required for neutralization, and the grid potential of tube I2 withrespect to its cathode is more negative than normal and produces anincrease in the impedance of the anodecathode path. The result is thatcircuit l5 absorbs less energy from the radio frequency tank circuit 2and more excitation is available for the modulated amplifier I, tendingto make the modulation characteristic more nearly coincide with thedotted straight line in Fig. 2. When the difference between the twocurves is (ii, the action is reversed so as to decrease the excitationto the modulated amplifier and again make the modulation characteristicfall closer to the straight line. At the points 0 of the modulationcharacteristic, of course, no such action takes place, since themodulation characteristic already lies on the straight line.

The oscillogram traced in Fig. 4 shows the improved form of themodulation characteristic after the compensating circuit of Fig. 1 hasbeen applied. The transmitter was operated under exactly the sameconditions of audio input, carrier output, radio frequency excitation,and input to the radio frequency exciting amplifier as when thecscillograms of Figs. 3 and 4 were taken. The greater linearity obtainedby means of the compensating circuit of my invention is clearly evident.

The oscillogram traced in Fig. 5 shows the relation of radio frequencyinput to the linear rectifier to its rectified output. This relation ishere shown to be substantially linear, indicating that the compensationobtained by means of the circuit of Fig. l is not a result ofcompensating distortion in the linear rectifier, but that the operationof the system is substantially as hereinbefore described.

The oscillogram traced in Fig. 6 shows the voltage effective betweengrid I21: and cathode I2c of the compensating modulator tube I2; (thehorizontal sweep circuit in this case was a 60 cycle wave). Thisoscillogram suggests another method of describing the operation of thecompensating system of my invention as applied in Fig. 1. Consider thevoltage wave shown on the oscillogram of Fig. 6 as comprising thedistortion components resulting from the modulation frequency impressedon the grid circuit of the modulated amplifier I. These distortioncomponents are then harmonics of the modulation frequency, and thecathode and grid of tube I2, operating under the respective potentialsimpressed thereon, constitute a harmonic selecting device, which cancelsthe undistorted component of the output voltage of the linear detector3, impressed with the distortion components on the grid I2b, with theundistorted input potential supplied through transformer 8 and impressedon the cathode I2c, leaving only the distortion components active in theoutput circuit of the tube I2. The tube I2, in this case, also acts toamplify the distortion components, and, in effect, to reintroduce themby means of the coupling coil I into the radio frequency excitationcircuit of the modulating amplifier I. in such phase as to oppose andreduce the distortion components generated in the modulation amplifier.

This aspect of phase relationship between the reintroduced harmonics andthe original modulating voltage gives rise to an important considerationin the design of such a compensating system. It is seen that unless thephase relationship is correct and remains so throughout the audiofrequency range of the transmitter, proper action of the compensatingsystem cannot be realized. No difficulty was experienced with thecircuit shown in Fig. 1, and it was found to operate satisfactorily overthe entire audio frequency range of the transmitter to substantiallyeliminate distortion in the output and effect linear operation of themodulation amplifier I.

In order to secure almost complete elimination of amplitude distortion,an amplifier stage may be inserted in the compensating modulator system.as shown in Fig. '7. Careful design of the amplifier stage is requiredin order that the phase relat onship of the opposing audio frequnecyvoltages might be maintained constant throughout the audio range of theamplifier.

Fig. '7 provides a three element electron tube 24m to select and amplifythe distortion components as described in connection with tube I2 inFig. 1, and an amplifier tube 25 to further amplify the distortioncomponents; and in addition, the output circuit of tube 25 is soarranged that the amplified distortion components in this circuit actdirectly through the modulation channel on the grid of the modulatedamplifier, in differentiation from the operation in Fig. 1 whereby thedistortion components are reintroduced through the radio frequencyexcitation channel to the grid of the modulated amplifier.

Fig. 8 shows the system of my invention embodied in the intermediateamplifier unit in a radio frequency transmitting system. The unitcomprises the modulated amplifier stage I, linear detector 3, andcompensation modulator stage I2, associated with the modulating energyinput system shown generally at 20, and a stage of the radio frequencyexcitation system indicated at 2I. The linear rectifier 3 is adapted tobe energized from the distorted output of a later stage throughconnection I9. The compensated output. of the unit is supplied toterminals 23.

The circuit arrangement for separating the distortion components fromthe undistorted components of the modulations may be, instead of thegrid-cathode circuit of an electron tube, as shown in Figs. 1, '7 and 8,a differential transformer, two electron tubes connected in opposition,or any device sensitive to the difference between two voltages,currents, or amounts of power, or to some function of the voltagecurrent or power. The distortion components after separation andamplification, may be reintroduced as a variation in voltage, current,power, resistance, or any other effect which can be used in any part ofthe circuit to vary the output of the modulating amplifier so as toreduce the distortion components.

Aside from the precaution against phase shift mentioned hereinbefore, itis also necessary that the entire compensating system, including thedetector 3 and the compensating modulator I2, and whatever circuitelements intervene and contribute to the final effect of thereintroduced harmonics on the modulated stage, must be capable ofpassing the high frequencies of the distortion components, which areseveral times as high as the modulation frequencies, to make the systemmost effective.

In addition, the whole compensating system should be free of amplitudedistortion in itself. If such is not the case, the compensating systemwill introduce distortion components of even higher frequency; that is,the original harmonics of the modulation frequency causing distortionwill be reduced, but harmonics of these harmonics will be generated.Although currents of these higher harmonics will generally be of lowamplitude and the total harmonic content of the output wave of thetransmitter will be reduced, weak but obj ectionably wide, spurioussi-debands may be generated.

I have found by experiment that the simple absorption method ofcompensation modulation as disclosed in the circuit of Fig. 1 wassubstantially linear in operation over the range corresponding to thelimited percent modulation capability required of the compensatingmodulator, and no trouble was experienced from this source. In thisconnection, it might be noted that the overall modulation capability ofthe compensating system need be only X%, for 100% modulation capabilityof the main modulator, where X is the sum of all the peak values, inpercent of the undistorted output, of the harmonics generated in themodulated amplifier; and that the reduction of distortion is dependentupon the overall gain in the compensating system from the value of theoriginal distortion components, selected as taken from the transmitteroutput, to the extent of their final effect on the same output, with therelations as given below:

Let the ideal modulation characteristic be the straight line y=a:v,where at: the input modu lating voltage and a, the amplification factorof the main modulation system. For simplicity, let us assume that theactual modulation characteristic with compensation is y=ax+bm where onlydistortion components of the second degree are considered present, and

is the distortion factor with compensation. Then the linear detectoroutput is caar-i-cba: where c is the linear detector coefficient. Now, 0is adso that ca=1 and the output of the linear detector, cazr-i-cbxbecomes with ca replaced by unity. Thus, the distortion componenteffective in the input circuit of the compensating modulator, bycounteracting the input modulating voltage component, is

YNOW, let K be the overall amplification of the compensating modulatorfrom its grid circuit to the final efiect on the output of the modulatedstage, including any intervening amplifiers. The amplified distortion isthen Since this has already been subtracted from the normal distortedoutput of the modulated amplifier to obtain the compensated outputazr-l-bx the uncompensated output would have been and the ratio ofcompensated to uncompensated distortion is rsl This equator,

shows the total reduction of distortion obtained when the gain of thecompensating device is times that of the modulation system. It is seenthat the equation is assymptotic, and that infinite amplification isrequired in the compensator to reduce the distortion to zero, but that areduction in the ratio of approximately 3 to 1 is readily obtained. Thisis approximately that shown by the difference in the oscillograms tracedin Figs. 3 and 4.

Another method for increasing the distortionreducing capability of thesystem of my invention is to reintroduce the distortion components byabsorption modulation of one of the lowerlevel radio frequency amplifierstages, with all radio frequency stages between this absorption stageand the modulated stage adjusted to act as linear amplifiers for a smallpercentage of modulation. This would require these stages to be operatedat cut-off bias as class B amplifiers, but they could be operated asclass C linear amplifiers by providing proper excitation and bias sothat they are linear over a certain range of vari ation of the radiofrequency excitation. This arrangement appears to offer considerablepossibilities, and is within the contemplation of my invention asdisclosed.

The compensating arrangement of my invention employs the audio-frequencymodulating voltage as introduced into the grid circuit of the modulatedamplifier, and does not compensate for distortion originating priorthereto in the audio-frequency pickup circuits. This is not a seriousproblem, however, since an audio amplifier of sufficient power for gridmodulation can be coupled into the grid-modulated circuit by means of atransformer designed to minimize amplitude distortion due to the flow ofgrid current in its secondary circuit. The design problems for such anaudio system are similar to those involved in driving Class B audioamplifiers and present no special diificulties. However, an audiodriving system involving compensation similar to that used in the tripletwin type of positive grid audio circuit might be used if desired.

Thus, while I have described .my invention in certain of its preferredembodiments, I desire that it be understood that modifications may bemade and that no limitations upon my invention are intended other thanmay be imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is as follows:

1. A system for reducing distortion in radio frequency grid-modulatedamplifiers due to the varying impedance to ground of the modulatedamplifier to the radio frequency excitation, which rovides acompensating load effectively in parallel with the varying impedance,said system comprising means for deriving the distortion components fromthe distorted modulated output of the modulated amplifier, and a radiofre quency energy absorbing circuit connected with said means and withground and coupled with the radio frequency excitation circuit, saidabsorbing circuit operative under the influence of said distortioncomponents to compensate by absorption in varying degrees the variationsin impedance of said modulated amplifier to said radio frequencyexcitation.

2. In a grid-modulated electron tube amplifier connected with a highfrequency excitation circuit and a low frequency modulation circuit,means for reducing distortion due to the varying impedance in theamplifier to high frequency excitation, said means comprising a highfrequency energy absorbing circuit coupled with the highfrequency'excitation circuit in parallel with the varying impedance inthe amplifier connected thereto, and means selectively energized bydistortion components in the modulated output of said amplifier andconnected with said absorbing circuit for controlling the operation ofsaid absorbing circuit for substantially compensating for the changes inimpedance in the said amplifier.

3. A system for reducing distortion in a radio frequency grid circuitmodulated amplifier due to the varying impedance to. ground of themodulated amplifier to the radio frequency excitation, which systemcomprises compensating load means connected effectively in parallel withthe varying impedance of the grid modulated amplifier, and means forcontrolling the operation of said lead means in step with the distortioncomponents of the modulation in the output of the modulated amplifier,the last said means including means energized from the output of saidmodulated amplifier and by the modulation components in the input ofsaid modulated amplifier for deriving the distortion components forcontrolling the aforesaid load means.

4. A system for reducing distortion in a radio frequency grid circuitmodulated amplifier due to the varying impedance to ground of themodulated amplifier to the radio frequency excitation, which systemcomprises a radio frequency energy absorbing circuit coupled with theradio frequency excitation circuit effectively in parallel with thevarying impedance of the grid modulated amplifier, and means forcontrolling the operation of said energy absorbing circuit to compensateby absorption in varying degrees the variations in impedance of saidmodulated amplifier to said radio frequency excitation, said meansincluding means energized from the output of said modulated amplifierand by the modulation components in the input of said modulatedamplifier for deriving the distortion components for controlling saidenergy absorbing circuit.

5. A system for reducing distortion in a radio frequency grid circuitmodulated amplifier due to the varying impedance to ground of themodulated amplifier to radio frequency excitation, which systemcomprises a compensating impedance load element connected effectively inparallel with the varying impedance of the grid modulated amplifier, andmeans for controlling the potential drop across said impedance loadelement in step with the variations in impedance of said modulatedamplifier to said radio frequency excitation, said means including meansenergized from the output of said modulated amplifier and by themodulation components in the input of said modulated amplifier forderiving the distortion components for controlling the potential dropacross said impedance load element.

6. In a high frequency grid circuit modulated amplifier, the method ofreducing distortion due to the varying impedance to ground of themodulated amplifier to radio frequency excitation, which consists inproviding a parallel impedance load of similar characteristics, andcontrolling the operation of the parallel impedance load with respect tothe radio frequency excitation in accordance with the varying impedanceof the modulated amplifier by deriving the distortion components fromthe output of the amplifier and utilizing them for controlling theparallel impedance load.

WALTER H. WIRKLER.

